Method and solution for pickling and cleaning metal



Patented May 16, 1939 PATENT orrlcs' ma'rnon am; sommon son momma arm cram G METAL Nelson E. Cook, Wheeling, W. Va.', assignor to Wheeling Steel Corporation, Wheeling, W. Va.,

a corporation of Delaware No Drawlng- Application June 23, 1936,

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This invention relates to the cleaning and picklingof metal articles, particularly iron and steel sheets andplates, prior to the application of a coating thereto, and especially for the prep aration of such articles for coating, with zinc.

It is, of course, well known [in the art that iron and steel sheets are usually annealed before the metal coating is applied and that in the process of annealing, a scale forms on'the surface ,of the sheet which consists primarily of compounds of iron and oxygen. This scale has first to be'removed and this iscommonly done by the immersion of the sheet in a pickling bath. The old standard method of cleaning sheets it has been to use solutions of either hydrochloric acid (H01) or sulphuric acid (H2804) in a solution of the desired concentration. .Such a so-.

lution first attacks the scale on theexterior of the sheets, reducing the scale to soluble iron salts, either chlorides or sulphates, depending upon which acid is used. After the reaction has proceeded far enough to sat through some of the scale, the acids then etch the base metal in accordance with the following typical reac- 85' absorption of the hydrogen causes considerable trouble. It is known in the industry as hydrogen absorption and it results in the lessening of the ductility of the coating onthe galvanized sheet and a lack of refinement in the coating.

I 40 The effect of hydrogen absorption may be prevented by the use in the pickling. solution 01' chemical inhibitors, which are usually of an organic nature. It is also. known that even if hydrogen absorption does occur, the hydrogen 45 may be substantially eliminated by any one of a'number of methods, such as a long time soaking? in water after pickling, or by the application'of heat after pickling. Such practices and methodswill take care of the ordinary iron 50 or steel sheet or article as heretofore produced.

Where the iron or steel sheets of other articles are rolled under very high pressures. as is now becoming a general practice, and especially when 58 a material having an oily base is used on the rolls in the rolling process, the nnished sheets. after ordinary annealing practice, which is applicable to sheets of this type. are much more susceptible to hydrogen absorption than sheets produced by the old style methods. As a matter 1 of fact, they are susceptible to such an extent that it is impossible to satisfactorily coat them by subjecting them to the standard method of pickling. My explanation for this difficulty is that withthls method of rolling, the grain struc-' 10 ture of the metal is very fine and the oily substance is' forced into the surface of the metal under very high pressures. This oily substance should be eliminated prior to coating, because if it were not eliminated prior to coating it would be released from the sheet at the high temperature in the coating bath and would make a good coating impossible dueto the "boiling up of the oily substance. In using any standard acid pickling practice where nascent hydrogen is 20 formed in the process, this hydrogen will be very readily absorbed by the fine structured steel or iron sheet, and in addition to this the oil in the pores of the sheet will be hydrogenated by reaction with the nascent hydrogen and will become a more or less fixed gelatinous substance which is extremely diillcult to remove from the poresof metal by known methods. I

In other words, hydrogen will be more or less permanently trapped and can only be released by extreme heat. .If this extreme heat happens to be the heat of the coating metal, then the coating will be of little value. Inhibitors which work satisfactorily to prevent hydrogen absorption with steel and iron sheets as formed by the old methods, do not satisfactorily control hydrogen absorption with sheets of the character under discussion, because when the sheet is immersed in the pickling bath, a film of the-inhibitor must form on the surface of the sheet. 40 This requires an appreciable length of time, and during this time hydrogen gas is formed, and in the case of very ilne-pored oily sheets, the time lapse is sumcient to permit objectionable hydrogen absorption.

In carrying out my present method, I have realized that it is necessary to accomplish at least three things, 1. e., 1) eliminate the oily organic substance from the pores of the sheet in order to provide a proper surface for good coating; (2) the oily substance must be removed, at the same time preventing hydrogen absorption or the trapping of hydrogen within the pores of the metal; and (3) the surface of the 5.

the type of coating desired.

According to the present invention, a pickling method is provided which accomplishes all of these results. A pickling solution is initially formed by adding to water, sulphuric acid, sodium nitrate, or an equivalent nitrate and a bichromate as typified by potassium bichromate. As a specific example, I may start by adding to the water 2% by weight of commercial sodium nitrate and approximately 1.5% by weight of sulphuric acid, and to this solution is then added approximately 3% by weight of ,commercial potassium bichromate.

The temperature of the bath is then brought to a point approximating F.

In the percentages above given, the sodium nitrate and sulphuric acid approximate a point of equilibrium, the excess being slightly in favor of the sodium nitrate. After the bath has been brought to a temperature of approximately 160 F. as described, the operation of pickling is proceeded with in the usual way. By adding bichromate to the pickling bath as above described, it is possible to pickle cold rolled sheets in such manner that they can be satisfactorily coated without further treatment, whereas when similar sheets have been pickled in a bath comprising sulphuric acid and sodium nitrate alone, the hydrogen has been absorbed into the sheets in such amounts that they cannot be subsequently satisfactorily coated. As the operation of pickling is continued, sulphuric acid and sodium nitrate are added from time to time, approximately equal parts by weight of sulphuric acid and sodium nitrate being added, and additional bichromate is also added from time to time as replenishment is required.

The reason why the addition of the bichromate to the pickling bath makes it possible to successfully pickle the cold rolled sheets without hydrogen absorption whereas the same bath without the bichromate will not operate to satisfactorily pickle cold rolled sheets, is not understood by me at the present time. I know, however, from the practice of the invention that the control of the bath at the temperature of pickling is very much more easily accomplished by reason of this addition of bichromate, and I advance some general observations in regard to the success of this meth- 0d. The sulphuric acid acts directly to remove annealing scale and etch the surface of the sheet. It reacts with the sodium nitrate to produce nitric acid which tends to remove oil from the surface of the sheet. To a lesser extent the sulphuric acid reacts with the potassium bichromate probably to form chromic acid, chromic acid being the aqueous solution of chromic trioxide. The chromic acid is a powerful oxidizing agent and readily attacks oily and organic matter in the pores of the sheet, oxidizing it to carbon dioxide which passes off in solution. This oxidizing agent is more eiiective than the nitric acid alone, and organic material which is embedded in the pores and which has previously absorbed hydrogen without the organic material itself being removed, is, with the present invention, completely removed. Whether this is actually the case or not cannot be definitely stated by me at the present time, but the results indicate that the bath is far superior than any heretofore developed for use in the pickling of cold rolled sheets.

While I have indicated certain proportions of sodium nitrate and sulphuric acid and potassium bichromate for initially starting the bath, this is sheet must be etched to a proper degree to take by way of illustration and the concentrations can be varied, it being preferable, however, to keep the same ratio of the different ingredients approximately.

In the foregoing description I have used sodium nitrate as a specific example of a material which may be employed because it is commercially the most available, but it will be understood that its chemical equivalent, such as ammonium and potassium nitrates, may be substituted. Potassium bichromate has been illustrated as a specific chromium salt, but other chromate salts may be substituted as for example, sodium bichromate or chromate salts of the various metals. Likewise, instead of using the chromate series of salts, any other salt may be employed which will react di rectly with sulphuric acid to form the oxy acids of the salt excluding sodium nitrate or other nitric acid producing salts, or form other salts of the oxy acids which, with further reaction will produce said oxy acids of the salts, including all such salts of the oxy acids and all such oxy acids formed thereby which will be oxidizing agents to aid in oxidizing and disposing of organic materials, as the oxy salts of the halogens, for example, potassium perchlorate, or of manganese, as for example, potassium permanganate, these being mentioned as typical oxy salts that may be used and not as being the only ones. According to the invention the sulphuric acid-sodium nitrate bath requires the addition of a third ingredient capable of reacting with sulphuric acid as above indicated, but this third material must be an ingredient which upon reaction with sulphuric acid will not also produce nitric acid. In otherwords. the invention cannot be accomplished without the use of an ingredient other than sulphuric aci: and sodium nitrate.

I claim:

1. A pickling solution comprising an aqueous solution of sulphuric acid and sodium nitrate and a bichromate salt in a concentration such that chromic acid along with nitric acid is produced in the pickling solution.

2. The method of pickling which comprises introducing metal articles to be pickled into a bath containing sulphuric acid and a nitrate, and containing also a chromate.

3. The method of pickling which comprises introducing metal articles such as sheets or plates to be pickled into a bath of water containing sulphuric acid and sodium nitrate and maintaining the temperature of the bath at a point approximating 160 F., and introducing into the bath a chromate compound capable of reaction with sulphuric acid.

4. The method of pickling sheets of ferrous metal which have been produced by rolling under high pressures on rolls lubricated with an oily organic substance, which comprises immersing the sheets in a bath containing sulphuric acid and sodium nitrate in an approximately equal amount and containing also a bichromate compound in a. lesser amount.

5. The method of pickling ferrous metal sheets of the character which have been produced by rolling under high pressure on rolls containing an introducing ferrous metal articles which have been produced in contact with oily substances into a bath containing sulphuric acid and a nitrate in a concentration approximating equilibrium 'and where sulphuric acid tends to produce nitric acid as an oxidizing agent by reaction with the nitrate and where sulphuric acid tends to etch the metal and loosen scale, and adding to the bath a smallarnount of compound capable of producing in the bath an oxidizing oxy-acid exclusive of nitric and sulphuric acids for oily substances in the metal to assure removal of the oily substance before it may absorb hydrogen released by the reaction of sulphuric acid and the metal being pickled and which prevents the sulphuric acid concentration from becoming excessive.

'7. The method of regulating a pickling bath which bath comprises an aqueous solution of sulphuric acid and a nitrate, and containing in addition an oxy-salt capable of reaction in the bath to produce an oxidizing compound other than and in addition to nitric acid which results from the reaction of the nitrate and the sulphuric acid.

NELSON E. COOK. 

